The invention relates to a method for dynamically diagnosing an exhaust gas probe disposed in an exhaust duct of an internal combustion engine, wherein the dynamic diagnosis is performed after a change in a lambda value of the exhaust gas, and on the basis of a comparison of a measured signal rise relative to an expected rise of the signal.
The invention further relates to a device for dynamically diagnosing an exhaust gas probe disposed in an exhaust duct of an internal combustion engine, the output signal thereof being fed to an engine controller connected to additional input signals providing at least additional information about intake air mass and fuel metering.
The storage capacity of an emission control system for oxygen is utilized for the purpose of receiving oxygen during lean phases and in turn dispensing oxygen during rich phases. As a result, oxidizable constituents of harmful gas contained in the exhaust gas can be converted. An exhaust gas probe disposed downstream of the emission control system thereby serves to monitor the oxygen storage capacity of said emission control system. The oxygen storage capacity has to be monitored within the scope of the on-board diagnostics system because said capacity represents a measurement for the conversion capacity of said emission control system. In order to determine said oxygen storage capacity, either the emission control system is initially saturated with oxygen in a lean phase and is subsequently emptied in a rich phase having an exhaust gas of a known lambda ratio with regard to the amount of exhaust gas passing through said emission control system or said system is initially emptied of oxygen in a rich phase and in a lean phase is subsequently filled with an exhaust gas of a known lambda ratio with regard to the amount of exhaust gas passing through said system. The lean phase is ended if the exhaust gas probe downstream of said emission control system detects the oxygen, which can no longer be stored by said emission control system. A rich phase is likewise ended if said exhaust gas probe detects the passage of rich exhaust gas. An output signal of said exhaust gas probe furthermore serves as additional information for a lambda control, which however primarily is based on an output signal of a lambda probe disposed upstream of said emission control system.
If the exhaust gas probe has aged, the output signal of the exhaust gas probe slows down in its reaction to changes in the exhaust gas composition, and deviations can result in the diagnosis of the emission control system, which can lead to an emission control system, which is no longer operating correctly, being mistakenly determined to be in working order. Tremendous importance is thus attached to a dynamic monitoring of the exhaust gas probe.
The evaluation of the reaction speed of the exhaust gas probe to a changing O2-concentration is understood by the term dynamic monitoring. Due to aging and contamination of the ceramic probe body, respectively the probe case, the possibility exists for the measurement of the O2 concentration of the exhaust gas to be considerably delayed. For this reason, the functions relevant to emissions, which require the lambda signal as an input signal, would be operated in a delayed fashion. As a result, emission threshold values can be exceeded, which have been specified by the lawmakers (European authorities or CARB, EPA).
A method of prior art for diagnosing an emission control system also evaluates, for example, the ratio of the amplitudes of the output signals of the lambda probe disposed upstream of the emission control system and the lambda probe disposed downstream of said system. An emission control system in working order dampens the amplitude of an oscillation of the oxygen content of the exhaust gas at the outlet of the internal combustion engine so that the ratio of the amplitudes upstream and downstream of said emission control system results in a high value. A delayed reaction of the exhaust gas probe disposed downstream of said emission control system leads, however, likewise to a reduction in the amplitude of the output signal thereof, whereby the oxygen storage capacity of said emission control system is evaluated as too high. An emission control system, which no longer meets the legal requirements, can thus under certain circumstances be mistakenly classified as being in good working order.
A dynamic diagnosis is made difficult because of the fact that the output signal of the exhaust gas probe is dependent upon the initial and the final lambda value when a rich to lean or lean to rich step change occurs. In addition, the influence of the emission control system described above must be taken into consideration, wherein the influences of temperature and the age of said emission control system must additionally be considered.
A method for dynamically diagnosing an exhaust gas probe is stated in the German patent publication DE 19722334. The exhaust gas probe is disposed in the exhaust gas downstream of an emission control system. The rate of change of an output signal of the exhaust gas probe is used as the evaluation criterion, said rate of change occurring, for example, after the beginning of a phase in overrun conditions. A disadvantage thereby is that this method only works when a high mass air flow (>>50 kg/h) occurs. This is because the influence of the catalytic converter can only then be disregarded. In such operating states, undesired conditions can, however, arise when returning to load conditions after being in the overrun mode.
In the German patent publication DE 10 2006 041 477 A1, a method for dynamically diagnosing an exhaust gas probe disposed in the exhaust duct of an internal combustion engine downstream of the emission control system is described, wherein the dynamic diagnosis is performed at the same time that a step change in the lambda value of the exhaust gas from rich to lean or from lean to rich occurs.
The present-day operation of dynamic monitoring calculates two O2 threshold values on the basis of the measured O2 concentration during a valid step load transition. The measured rise time of the O2 concentration from the first to the second threshold value is used as the evaluation criterion for the dynamic characteristics of the exhaust gas probe. If the measured rise time remains under a fixed threshold value, an intact message results, otherwise a fault is reported. The lambda signal is thereby qualified with respect to a fixed value in an operating range to be applied.
A disadvantage in this is that a setting of the operating range is dependent upon the component tolerances of the components upstream of the emission control system, including sensors and actuators. Possible drifting of the component characteristics is not taken into account with fixed threshold values. In addition, with these fixed threshold values, only a limited operating range for changes in load can be used for a dynamic diagnosis. Dynamic characteristics of the exhaust gas probe can also not always be correctly diagnosed with fixed threshold values. As a result, a dynamically defective exhaust gas probe can be evaluated as being in working order, which should be considered as critical in light of the massively increased or increasing legal requirements.